Water, Weak Interactions, pH and Buffers: every key term you need (+ practice quiz)
25 flashcard terms for Biochemistry Topic 1, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
A weak attraction between a hydrogen already bonded to an electronegative atom and a nearby lone pair. Individually weak, but in bulk they set water's boiling point and hold biological structures together.
Hydrophobic effect
Nonpolar groups cluster in water not because they attract each other but because burying them releases ordered water molecules, raising entropy. This is the dominant force in protein folding and membrane assembly.
Van der Waals interaction
A brief attraction between transient dipoles in adjacent atoms. It is only significant at close packing distance, which is why a tightly fitted binding site binds far better than a loose one.
Ionic interaction in water
An attraction between opposite charges, strongly weakened in aqueous solution because water's high dielectric constant screens charge. Salt bridges therefore matter most when buried away from solvent.
Amphipathic molecule
A molecule carrying both a polar region and a nonpolar region. Phospholipids and detergents are amphipathic, which is why they self-assemble into bilayers and micelles.
Micelle
A spherical aggregate in which nonpolar tails point inward and polar heads face water. It forms above a characteristic concentration and is how single-chain amphipaths hide their tails.
Osmolarity
The total concentration of dissolved particles that cannot cross a membrane. Water moves toward the higher value, which is why cells actively control internal solute number.
Colligative property
A solution property that depends on the number of dissolved particles rather than their identity, such as freezing point depression and osmotic pressure.
Ionization of water
Water dissociates slightly into hydronium and hydroxide ions. The product of their concentrations is a fixed constant at a given temperature, which links pH and pOH.
pH
The negative logarithm of hydrogen ion concentration. Because the scale is logarithmic, one unit is a tenfold change in acidity, and small shifts have large biological effects.
Acid dissociation constant
The equilibrium constant for a weak acid releasing its proton. A larger value means a stronger acid and a lower pKa.
pKa
The pH at which a titratable group is exactly half protonated. Comparing pH with pKa is the fastest way to predict whether a group carries charge in a given environment.
Henderson-Hasselbalch equation
Relates pH to pKa and the log ratio of conjugate base to acid. It converts a titration question into arithmetic and predicts the direction of any protonation shift.
Buffer
A mixture of a weak acid and its conjugate base that resists pH change by absorbing added protons or hydroxide. Capacity is greatest within about one pH unit of the pKa.
Buffering capacity
The amount of strong acid or base a buffer can absorb before pH swings sharply. It rises with total buffer concentration and falls as pH moves away from the pKa.
Titration curve
A plot of pH against added base. Its flat inflection region marks the buffering zone, and its midpoint identifies the pKa of the group being titrated.
Polyprotic acid
An acid with more than one ionizable proton, releasing them in sequence with separate pKa values and producing a titration curve with multiple plateaus.
Bicarbonate buffer system
The main extracellular blood buffer, coupling dissolved carbon dioxide to bicarbonate. It is unusually effective because respiration can adjust the acid component in real time.
Phosphate buffer system
The main intracellular buffer, using the dihydrogen and monohydrogen phosphate pair whose pKa sits near physiological pH.
Zwitterion
A molecule carrying both a positive and a negative charge with no net charge overall. Free amino acids exist in this form at neutral pH.
Isoelectric point
The pH at which a molecule has zero net charge and therefore stops migrating in an electric field. It is the basis of isoelectric focusing.
Condensation reaction
Joins two molecules with the loss of water. Peptide bonds, glycosidic bonds and ester bonds in lipids are all made this way.
Hydrolysis
Cleaves a bond by adding water. Thermodynamically favourable for most biological polymers, which is why enzymes rather than stability control their turnover.
Standard free energy change
The free energy change under defined standard conditions. It fixes the equilibrium constant but says nothing about rate, and real cellular direction depends on actual concentrations.
Coupled reaction
Drives an unfavourable process by linking it to a strongly favourable one, usually the hydrolysis of a phosphoanhydride bond, so the summed free energy change is negative.